Fertilizer is a substance added to soil to help plants grow stronger and faster. Just as you need nutritious food to grow healthy and strong, plants need certain nutrients from the soil. The three most important nutrients are nitrogen (which helps leaves grow green and lush), phosphorus (which supports strong roots and flowers), and potassium (which keeps plants healthy overall). When farmers plant the same crops year after year, or when gardeners grow vegetables in containers, the soil gradually runs out of these nutrients. Fertilizer replenishes what's missing.

Some fertilizers come from natural sources: composted food scraps, aged manure from farm animals, or ground-up fish. Others are manufactured in factories by combining chemicals. A farmer spreading fertilizer across a cornfield is essentially feeding the soil so it can feed the plants. Home gardeners might sprinkle fertilizer around their tomato plants or mix it into potting soil.

The word can also be used metaphorically. If someone says a challenging book was fertilizer for the mind, they mean it helped their thinking grow and flourish, just as fertilizer helps plants thrive.